1. Field of the Invention
The present invention relates generally to infra-red (IR) transmitting windows used to protect IR sensors, and, more particularly, to such IR transmitting windows having thereon a metallic grid to provide shielding of the IR sensors against electromagnetic interference (EMI) or low observability.
2. Description of Related Art
A conductive metallic grid is desired on the exterior surface of an IR-transmitting window to provide EMI shielding or low observability. However, these grids and the antireflection coating covering them are partially or completely damaged in rain erosion tests. The combination antireflection/rain erosion coatings which are deposited on top of the grid-covered window are highly stressed, since they must conform to the non-flat surface. Consequently, these coatings are easily damaged by the rain erosion test. Furthermore, the grids are removed due in part to the poor adhesion of the metal grid lines to the window surface.
When subjected to a rain erosion environment at normal angle, the IR transmission is reduced by more than 5%, which is unacceptable according to industry standards. This reduction in IR transmission is primarily caused by partial removal of the antireflection coating, in addition to some substrate microcracks which occur at the surface of the window.
In one attempt to solve these problems, buried metallic grids have been produced by depositing an additional protective layer of the window material over the top of the grid, followed by a grinding and polishing of the additional layer. This approach is very expensive, requiring an additional deposition step and further processing steps, all of which are time-consuming and require additional IR-transmitting window material.
Thus, an IR-transmitting window that can survive rain and sand erosion and that can be fabricated with a minimum number of additional steps is required.